1. Field of the Invention
The present invention relates generally to an apparatus and method for loading a medical device onto a minimally invasive delivery system, such as a delivery catheter, and deploying the device in situ.
2. Description of the Related Art
Percutaneous aortic valve replacement (PAVR) technology is emerging that provides an extremely effective and safe alternative to therapies for aortic stenosis specifically, and aortic disease generally. Historically, aortic valve replacement necessitated surgery with its attendant risks and costs. The replacement of a deficient cardiac valve performed surgically requires first opening the thorax, placing the patient under extracorporeal circulation or peripheral aorto-venous heart assistance, temporarily stopping the heart, exposing and excising the deficient valve, and then implanting a prosthetic valve in its place. This procedure has the disadvantage of requiring prolonged patient hospitalization, as well as extensive and often painful recovery. Although safe and effective, surgical replacement presents advanced complexities and significant costs. For some patients, however, surgery is not an option for one or many possible reasons. As such, a large percentage of patients suffering from aortic disease go untreated.
To address the risks associated with open-heart implantation, devices and methods for replacing a cardiac valve by less invasive means have been developed. For example, CoreValve, Inc. of Irvine, Calif. has developed a prosthetic valve fixed to a collapsible and expandable support frame that can be loaded into a delivery catheter. Such a prosthesis may be deployed minimally invasively through the vasculature at significantly less patient risk and trauma. A description of the CoreValve bioprosthesis and various embodiments appears in U.S. Pat. Nos. 7,018,406 and 7,329,278, and published Application Nos. 2004/0210304 and 2007/0043435. By using a minimally invasive replacement cardiac valve, patient recovery is greatly accelerated over surgical techniques. In the case of the CoreValve device, the support frame is made from shape memory material such as Nitinol. Other catheter-delivery valve replacement systems use stainless steel, or do not rely upon a rigid frame.
As demonstrated successfully to date, using a transcatheter procedure, percutaneous aortic valve replacement proceeds by delivering a prosthetic valve to the diseased valve site for deployment, either using a balloon to expand the valve support against the native lumen or exposing a self-expanding support in situ and allowing it to expand into place. With the latter, the self-expanding frame remains sheathed during delivery until the target site is reached. Advantageously, the frame may be secured to the catheter to avoid premature deployment as the sheath is withdrawn. In the CoreValve valve prosthesis, a hub is employed with two lateral buttons around each of which a frame zig may reside during delivery. The internal radial force of the sheath keeps the frame compressed against the catheter, including the frame zigs in place around the lateral buttons. The catheter generally comprises at least two tubes, an inner tube that carries the prosthesis and an outer tube that carries the sheath, permitting the sheath to move relative to the prosthesis.
As with traditional cardiovascular interventional therapies, transcatheter device deployment may proceed retrograde against normal blood flow, or antegrade, with blood flow. For aortic valve replacement, entry through the femoral arteries proceeds in a retrograde format through the iliac, descending aorta, over the arch and to the native annulus. In some cases, entry has been made closer to the arch; for example through the left subclavian artery. Antegrade procedures have been performed whether delivery takes place through the venous system transeptally to the native aortic annulus. More recently, transapical procedures have been performed whereby a cardiac surgeon delivers a catheter through the left ventricle apex to the target site.
With retrograde deployment, it is generally desired that the catheter be advanced within the vasculature so that the device is positioned where desired at the annulus site. With some embodiments under development, the desired site is the annulus itself. With the CoreValve device, the desired site extends from the annulus to the ascending aorta, given its relative length. In the transfemoral approach, when the CoreValve device is positioned at the desired site, the sheath is withdrawn to the point where the inflow end of the device (preferably positioned at the native annulus) expands to engage and push radially outwardly the native valve leaflets. The sheath continues to be withdrawn proximally as the prosthesis continues to expand as it is exposed until the sheath covers just the outflow portion of the prosthesis still secured to the hub ears. Any readjustment of the axial position of the device in situ can be made during this process based upon electronic visual feedback during the procedure. Once well positioned, the sheath is fully withdrawn, the device fully expands in place, and the catheter is withdrawn through the center of the device and out through the vasculature. While it would be possible to deploy the prosthetic device such that the sheath could be withdrawn distally so that the outflow end of the prosthesis deploys first, such an arrangement would require advancing distally the outer tube of the catheter connected to the sheath distally. In the case of transfemoral retrograde delivery, that would cause the outer tube to project well into the left ventricle, which is not desirable. In a antegrade approach, for example transapical delivery, the reverse situation exists. There it is more desirable to advance the sheath distally to expose the inflow end of the prosthesis at the native annulus first. The native anatomy can accommodate this distal deployment because the outer tube carrying the sheath is advanced up the ascending aorta towards the arch. Like the retrograde approach, once the valve prosthesis is fully deployed, the catheter may be withdrawn through the center of the prosthesis and removed through the apex of the heart.
With minimally invasive cardiac valve replacement, as may be appreciated, loading of a self-expanding valved frame into a sheath (or capsule) can be difficult because of the frictional forces that inhibit movement of the frame into and out of the sheath. The radial forces attendant in a self-expanding frame are pushing the frame against the inner wall of the sheath during the axial movement of the frame relative to the sheath. The friction translates into a greater axial force that must be applied to smooth and reliably load and deploy the frame from within the distal sheath. Where precision is demanded, such friction requiring greater axial force to be applied makes accurate deployment more difficult. Accordingly, a need exists for a suitable system and method of loading and deploying a self-expanding valved frame using a delivery catheter that reduces the inhibiting nature of the frictional forces during loading and deployment.